California’s overdue for a ‘ground-rupturing’ earthquake. When, where will it hit?

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Believe it or not, the last 100 years have been relatively quiet for earthquakes among California’s most active faultlines. Expect that to change.

A new study from researchers at the U.S. Geological Survey found a notable decline in the frequency of “ground-rupturing” earthquakes, defined as magnitude 6.5 or higher, over the past century in California along the San Andreas, San Jacinto and Hayward Faults.

“Whether due to a statistical anomaly, some longer‐term modulation of earthquake occurrence, or another cause, our results emphasize that the hiatus of the last century has been exceptional,” according to the abstract for a study published in the journal “Seismological Research Letters.”

Researchers studied the seismic record for three faults over the last 1,000 years and found “that the gap was very unlikely — along the lines of a 0.3% chance of occurring...”

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“If our work is correct, the next century isn’t going to be like the last one, but could be more like the century that ended in 1918,” said USGS researcher Glenn Biasi in a statement.

The study found there were eight large, ground-rupturing earthquakes along those three faults between 1800 and 1918, including the 1906 San Francisco earthquake that killed hundreds and left hundreds of thousands homeless.

Since 1918, the most notable earthquake along the San Andreas Fault was the 1989 Loma Prieta Earthquake, which was magnitude 6.9. Neither the San Jacinto nor Hayward faults have registered a temblor that size or larger in the last 100 years.

“We know these big faults have to carry most of the (tectonic) motion in California, and sooner or later they have to slip,” Biasi said. “The only questions are how they’re going to let go and when.”

So what’s behind the calm before the seismic storm?

Researchers don’t know, but they do know that this isn’t a statistical error.

“We’re saying, no, it’s not a data problem, it’s not a data choice problem, it doesn’t matter how you slice this,” Biasi said. “We just have not had earthquakes that past records predict that we should have had.”

In a statement, Biasi likened the situation to driving on the freeway and noticing that there aren’t any other cars on the road. It could be a statistical fluke, he said, but if traffic stays light long enough, “the other reason there might be no cars is that up around the bend, there’s a wreck.”

The study authors urged more seismologists to study the reason for the “wreck around the bend” that led to the exceptional pause of the last 100 years.

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